In order to reduce NOx in the exhaust gas of an engine of an automobile vehicle, an engine is provided with an exhaust gas recirculation system recirculating the exhaust gas to an intake side. An exhaust gas recirculation passage thereof is provided with an EGR valve (“Exhaust Gas Recirculation” valve), and an EGR valve is controlled based on driving information or the like of an engine. If high-temperature exhaust gas is recirculated to an intake side as it is, the efficiency of an engine becomes deteriorated. Thus, in general, an exhaust gas recirculation passage is provided with an EGR cooler for cooling high-temperature exhaust gas. An EGR cooler is disclosed in Patent Document 1, for example.
Patent Document 1: JP-A-2003-184659
Exhaust gas of an engine contains sulfur oxides coming from a fuel system, and nitrate oxides and particulate matter (PM) such as soot and the like which are produced by combustion. Sulfur oxides and nitric acid compounds produce corroding matter containing sulfuric acid and nitric acid in an exhaust gas passage. When such corroding matter enters an EGR valve, the matter corrodes a portion of a valve mechanism exposed to exhaust gas passing through an exhaust passage of the EGR valve and the inner surface of the exhaust gas passage of the valve body. The tendency is accelerated by the temperature of an exhaust gas being reduced by an EGR cooler.
An exhaust gas passage of an EGR valve is made of material such as cast iron or aluminum alloys, and thus corroding matter produced in an exhaust gas passage corrodes the inner surface of the exhaust gas passage, and produces corroded products such as rust and the like. Such corroded products can flake off by the flow of circulating exhaust gas and the vibration or the like of an engine. Meanwhile, a valve head (valve disk) of an EGR valve is made of material such as stainless steel or the equivalent that resists corrosion; however, the corroding matter corrodes a portion of the surface of the valve head extremely near to the superficial layer of the surface thereof and the surface thereof is put into a rough state. Corroded products flaked off from the corroded inner surface of an exhaust gas passage, the soot contained in exhaust gas, or the like come to easily adhere to such an surface of the valve head. Those corroded products, soot, and the like also adhere to the vicinity of a bearing section of a valve rod, and become a cause of interfering with an operation between the valve rod and the bearing.
The present invention is made in view of such a technical situation, and an object of the present invention is to reduce the tendencies of an exhaust gas passage and a valve rod of an EGR valve provided in an exhaust gas recirculation passage to be adhered to by corroded products, soot and the like.